I've lived around the corner from this place for over a year and was always hesitant about going in based on what it looks like from the street. I've wasted a year with inferior taco shops!
The service is friendly, and it may take longer to get your food than at other places, but it's worth it. I can't say that the fish tacos are the best I've ever had, but they are only overshadowed by those I got in Cabo San Lucas.
I'm a big guy, with a big guy's appetite, and I got absolutely stuffed here for less than $9, which included 2 cans of soda (at 75 cents each).
Tacos Atoyac is as close to Mexico City street food as I've seen anywhere outside of Mexico City. Don't be thrown off by the big gringo at the counter, this T.A. is authentic Oaxaca in north Phoenix.